That she has many friends. (being the tallest girl is not the most important point because its added as a description of Emily. It is not the main focus of the sentence)
The words after the two first commas are just words to express her sister and, even if you perceive these words out of the sentence, the structure of the sentence is still present. The other parts are all supporting information. They are not giving the main information and are not describing the central theme.
